Electric variable-resistance devices, commonly known as potentiometers, are known in the art, and are adapted to be connected to an electric circuit to control its electrical quantities by means of a variable resistor.
As a rule, these devices are used, without limitation, as voltage dividers or current regulators, for regulating the electric current that circulates in a circuit.
A particular class of potentiometers includes an electric adjustment device of the rotary type, comprising a shaft that interacts with the variable resistor and rotates about its axis.
The electric adjustment device is in turn held within a tubular housing, whereas the shaft is attached to a knob that is designed to be rotated by a user.
Since these devices are often designed to be installed in particularly harsh environments, such as engine rooms or outdoors, they must be protected from infiltrations of dirt particles, liquids and possibly oxidizing or corrosive substances that might affect their efficiency.
Furthermore, these devices are often exposed to shocks and excessive vibrations that might cause damages to or failures of the moving parts, whereby appropriate protection means should be provided as they are being handled.
DE102008048470 discloses a rotary potentiometer having a housing for receiving the resistor and a knob for engaging the portion of the shaft that projects out of the housing to allow a user to rotate it. This prior art potentiometer also has limit stop means associated with the knob and the housing to restrict the angular range of motion of the shaft and prevent the moving parts of the resistor from violently impacting abutment surfaces.
Furthermore, the device comprises a sealing ring with a central opening for the passage of the shaft, which is adapted to seal the electric adjustment device and prevent infiltration of solid or liquid materials from the installation environment into the housing.
The main drawback of this prior art solution is that a considerable portion of the shaft is external to the sealing element and is not adequately protected against penetration of dirt particles or infiltration of liquids.
Therefore, while this potentiometer ensures adequate protection to its electric parts, it still does not ensure a proper sealing effect against impurities, liquids or corrosive substances that might penetrate the knob.
DE1020080484470, GB940308, EP1691380, DE1980281 and DE8805076 disclose protection devices for electric potentiometers that have all the features as set out in the preamble of claim 1.
Nevertheless, these protection devices are relatively complex and expensive to build, and only partially limit the infiltration of dust or impurities external to the electric potentiometer.